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The present invention provides a heat-treated steel material having
strength of 1.800 GPa or more with obtaining excellent toughness and
weldability. The heat-treated steel material includes a chemical
composition represented by, in mass %: C: 0.05% to 0.30%; Mn: 2.0% to
10.0%; Cr: 0.01% to 1.00%; Ti: 0.010% to 0.100%; B: 0.0010% to 0.0100%;
Si: 0.08% or less; P: 0.050% or less; S: 0.0500% or less; N: 0.0100% or
less; Ni: 0% to 2.0%; each of Cu, Mo, and V: 0% to 1.0%; each of Al and
Nb: 0% to 1.00%; and the balance: Fe and impurities.
"4612.times.[C]+102.times.[Mn]+605.gtoreq.1800" is satisfied where [C]
denotes a C content and [Mn] denotes a Mn content. The heat-treated steel
material includes a microstructure in which 90 volume % or more is formed
of martensite, and a dislocation density in the martensite is equal to or
more than 9.0.times.10.sup.15 m.sup.-2.

1. A heat-treated steel material, comprising: a chemical composition
represented by, in mass %: C: 0.05% to 0.30%; Mn: 2.0% to 10.0%; Cr:
0.01% to 1.00%; Ti: 0.010% to 0.100%; B: 0.0010% to 0.0100%; Si: 0.08% or
less; P: 0.050% or less; S: 0.0500% or less; N: 0.0100% or less; Ni: 0.0%
to 2.0%; Cu: 0.0% to 1.0%; Mo: 0.0% to 1.0%; V: 0.0% to 1.0%; Al: 0.00%
to 1.00%; Nb: 0.00% to 1.00%; and the balance: Fe and impurities, and a
microstructure represented by martensite: 90 volume % or more, wherein an
"Expression 1" is satisfied where [C] denotes a C content (mass %) and
[Mn] denotes a Mn content (mass %),
4612.times.[C]+102.times.[Mn]+605.gtoreq.1800 "Expression 1"; wherein a
dislocation density in the martensite is equal to or more than
9.0.times.10.sup.15 m.sup.-2; and wherein a tensile strength is 1.800 GPa
or more.

2. The heat-treated steel material according to claim 1, wherein in the
chemical composition, Ni: 0.1% to 2.0%, Cu: 0.1% to 1.0%, Mo: 0.1% to
1.0%, V: 0.1% to 1.0%, Al: 0.01% to 1.00%, or Nb: 0.01% to 1.00%, or any
combination thereof is satisfied.

3. A method of manufacturing a heat-treated steel material, comprising:
heating a steel sheet to a temperature zone of not less than an Ac.sub.3
point nor more than "the Ac.sub.3 point +200.degree. C." at an average
heating rate of 10.degree. C./s or more; next, cooling the steel sheet
from the temperature zone to an Ms point at a rate equal to or more than
an upper critical cooling rate; and next, cooling the steel sheet from
the Ms point to 100.degree. C. at an average cooling rate of 50.degree.
C./s or more, wherein the steel sheet comprises a chemical composition
represented by, in mass %: C: 0.05% to 0.30%; Mn: 2.0% to 10.0%; Cr:
0.01% to 1.00%; Ti: 0.010% to 0.100%; B: 0.0010% to 0.0100%; Si: 0.08% or
less; P: 0.050% or less; S: 0.0500% or less; N: 0.0100% or less; Ni: 0.0%
to 2.0%; Cu: 0.0% to 1.0%; Mo: 0.0% to 1.0%; V: 0.0% to 1.0%; Al: 0.00%
to 1.00%; Nb: 0.00% to 1.00%; and the balance: Fe and impurities, wherein
an "Expression 1" is satisfied where [C] denotes a C content (mass %) and
[Mn] denotes a Mn content (mass %),
4612.times.[C]+102.times.[Mn]+605.gtoreq.1800 "Expression 1".

4. The method of manufacturing the heat-treated steel material according
to claim 3, wherein in the chemical composition, Ni: 0.1% to 2.0%, Cu:
0.1% to 1.0%, Mo: 0.1% to 1.0%, V: 0.1% to 1.0%, Al: 0.01% to 1.00%, or
Nb: 0.01% to 1.00% or any combination thereof is satisfied.

5. The method of manufacturing the heat-treated steel material according
to claim 3, wherein the steel sheet is subjected to forming before the
temperature of the steel sheet reaches the Ms point after the heating the
steel sheet to the temperature zone of not less than the Ac.sub.3 point
nor more than "the Ac.sub.3 point +200.degree. C.".

6. The method of manufacturing the heat-treated steel material according
to claim 4, wherein the steel sheet is subjected to forming before the
temperature of the steel sheet reaches the Ms point after the heating the
steel sheet to the temperature zone of not less than the Ac.sub.3 point
nor more than "the Ac.sub.3 point +200.degree. C.".

Description

TECHNICAL FIELD

[0001] The present invention relates to a heat-treated steel material used
for an automobile and the like, and a method of manufacturing the same.

BACKGROUND ART

[0002] A steel sheet for automobile is required to improve fuel efficiency
and crashworthiness. Accordingly, attempts are being made to increase
strength of the steel sheet for automobile. However, ductility such as
press formability generally decreases in accordance with the improvement
of strength, so that it is difficult to manufacture a component having a
complicated shape. For example, in accordance with the decrease in
ductility, a portion with a high working degree fractures, or springback
and wall warp become large to deteriorate accuracy in size. Therefore, it
is not easy to manufacture a component by press-forming a high-strength
steel sheet, particularly, a steel sheet having tensile strength of 780
MPa or more.

[0003] Patent Literatures 1 and 2 describe a forming method called as a
hot stamping method having an object to obtain high formability in a
high-strength steel sheet. According to the hot stamping method, it is
possible to form a high-strength steel sheet with high accuracy, and a
steel material obtained through the hot stamping method also has high
strength. Further, a microstructure of the steel material obtained
through the hot stamping method is substantially made of a martensite
single phase, and has excellent local deformability and toughness
compared to a steel material obtained by performing cold forming on a
high-strength steel sheet with multi-phase structure.

[0004] Generally, crushing strength when collision of an automobile occurs
greatly depends on material strength. For this reason, in recent years, a
demand regarding a steel material having tensile strength of 1.800 GPa or
more, for example, has been increasing, and Patent Literature 3 describes
a method having an object to obtain a steel material having tensile
strength of 2.0 GPa or more.

[0005] According to the method described in Patent Literature 3, although
it is possible to achieve the desired object, sufficient toughness and
weldability cannot be obtained. Even with the use of the other
conventional techniques such as steel sheets described in Patent
literatures 4 to 6, and the like, it is not possible to obtain tensile
strength of 1.800 GPa or more while achieving excellent toughness and
weldability.

[0012] The present invention has an object to provide a heat-treated steel
material capable of obtaining tensile strength of 1.800 GPa or more while
achieving excellent toughness and weldability, and a method of
manufacturing the same.

Solution to Problem

[0013] As a result of earnest studies to solve the above problems, the
present inventors found out that when a heat-treated steel material
contains specific amounts of C and Mn, it is possible to obtain strength
of 1.800 GPa or more with obtaining excellent toughness and weldability,
although details thereof will be described later.

[0014] The higher a C content, the higher a dislocation density in
martensite and finer substructures (lath, block, packet) in a prior
austenite grain. Based on the above description, it is considered that a
factor other than solid-solution strengthening of C also greatly
contributes to the strength of martensite. The mechanism by which
dislocation occurs in the martensite and the mechanism by which the
substructures become fine, is estimated as follows. Transformation from
austenite to martensite is accompanied by expansion, so that in
accordance with martensite transformation, strain (transformation strain)
is introduced into surrounding non-transformed austenite, and in order to
lessen the transformation strain, the martensite right after the
transformation undergoes supplemental deformation. On this occasion,
since the transformation strain in austenite strengthened by C is large,
fine lath and block are generated to reduce the transformation strain,
and the martensite undergoes supplemental deformation while being
subjected to introduction of a large number of dislocations. It is
estimated that, because of such mechanisms, the dislocation density in
the martensite is high, and the substructures in the prior austenite
grain become fine.

[0015] The present inventors found out, based on the above-described
estimation, that the dislocation density increases, crystal grains become
fine, and the tensile strength dramatically increases, in accordance with
quenching, also when a steel sheet contains Mn, which introduces a
compressive strain into a surrounding lattice similarly to C.
Specifically, the present inventors found out that when a heat-treated
steel material including martensite as its main structure contains a
specific amount of Mn, the steel material is affected by indirect
strengthening such as dislocation strengthening and grain refinement
strengthening, in addition to solid-solution strengthening of Mn,
resulting in that desired tensile strength can be obtained. Further, it
has been clarified by the present inventors that in a heat-treated steel
material including martensite as its main structure, Mn has strengthening
property of about 100 MPa/mass % including the above-described indirect
strengthening.

[0016] It has been conventionally considered that the strength of
martensite mainly depends on the solid-solution strengthening property of
C, and there is no influence of an alloying element almost at all (for
example, Leslie et al., Iron & Steel Material Science, Maruzen, 1985), so
that it has not been known that Mn exerts large influence on the
improvement of strength of the heat-treated steel material.

[0017] Then, based on these findings, the inventors of the present
application reached the following various embodiments of the invention.

[0075] The method of manufacturing the heat-treated steel material
according to (3), wherein in the chemical composition,

[0076] Ni: 0.1% to 2.0%,

[0077] Cu: 0.1% to 1.0%,

[0078] Mo: 0.1% to 1.0%,

[0079] V: 0.1% to 1.0%,

[0080] Al: 0.01% to 1.00%, or

[0081] Nb: 0.01% to 1.00% or

[0082] any combination thereof is satisfied.

[0083] (5)

[0084] The method of manufacturing the heat-treated steel material
according to (3) or (4), wherein the steel sheet is subjected to forming
before the temperature of the steel sheet reaches the Ms point after the
heating the steel sheet to the temperature zone of not less than the
Ac.sub.3 point nor more than "the Ac.sub.3 point +200.degree. C.".

Advantageous Effects of Invention

[0085] According to the present invention, it is possible to obtain
strength of 1.800 GPa or more with obtaining excellent toughness and
weldability.

DESCRIPTION OF EMBODIMENTS

[0086] Hereinafter, an embodiment of the present invention will be
described. Although details will be described later, a heat-treated steel
material according to the embodiment of the present invention is
manufactured by quenching a specific steel sheet for heat treatment.
Therefore, hardenability of the steel sheet for heat treatment and a
quenching condition exert influence on the heat-treated steel material.

[0087] First, a chemical composition of the heat-treated steel material
according to the embodiment of the present invention and the steel sheet
for heat treatment used for manufacturing the heat-treated steel material
will be described. In the following description, "%" being a unit of
content of each element contained in the heat-treated steel material and
the steel sheet used for manufacturing the heat-treated steel material
means "mass %" unless otherwise mentioned. The heat-treated steel
material according to the present embodiment and the steel sheet used for
manufacturing the heat-treated steel material includes a chemical
composition represented by C: 0.05% to 0.30%, Mn: 2.0% to 10.0%, Cr:
0.01% to 1.00%, Ti: 0.010% to 0.100%, B: 0.0010% to 0.0100%, Si: 0.08% or
less, P: 0.050% or less, S: 0.0500% or less, N: 0.0100% or less, Ni: 0.0%
to 2.0%, Cu: 0.0% to 1.0%, Mo: 0.0% to 1.0%, V: 0.0% to 1.0%, Al: 0.00%
to 1.00%, Nb: 0.00% to 1.00%, and the balance: Fe and impurities, and an
"Expression 1" is satisfied where [C] denotes a C content (mass %) and
[Mn] denotes a Mn content (mass %). Examples of the impurities are those
contained in a raw material such as an ore or scrap, and those contained
during manufacturing processes.

4612.times.[C]+102.times.[Mn]+605.gtoreq.1800 "Expression 1";

[0088] (C: 0.05% to 0.30%)

[0089] C is an element that enhances hardenability of the steel sheet for
heat treatment and improves strength of the heat-treated steel material.
If the C content is less than 0.05%, the strength of the heat-treated
steel material is not sufficient. Thus, the C content is 0.05% or more.
The C content is preferably 0.08% or more. On the other hand, if the C
content exceeds 0.30%, the strength of the heat-treated steel material is
too high, resulting in that toughness and weldability significantly
deteriorate. Thus, the C content is 0.30% or less. The C content is
preferably 0.28% or less, and more preferably 0.25% or less.

[0090] (Mn: 2.0% to 10.0%)

[0091] Mn is an element which enhances the hardenability of the steel
sheet for heat treatment. Mn strengthens martensite through not only
solid-solution strengthening but also facilitation of introduction of a
large number of dislocations during martensite transformation, which
occurs when manufacturing the heat-treated steel material. Specifically,
Mn has an effect of facilitating the dislocation strengthening. Mn
refines substructures in a prior austenite grain after the martensite
transformation through the introduction of dislocations, to thereby
strengthen the martensite. Specifically, Mn also has an effect of
facilitating grain refinement strengthening. Therefore, Mn is a
particularly important element. If the Mn content is less than 2.0% where
the C content is 0.05% to 0.30%, the effect by the above function cannot
be sufficiently obtained, resulting in that the strength of the
heat-treated steel material is not sufficient. Thus, the Mn content is
2.0% or more. The Mn content is preferably 2.5% or more, and more
preferably 3.6% or more. On the other hand, if the Mn content exceeds
10.0%, the strength of the heat-treated steel material is too high,
resulting in that toughness and hydrogen embrittlement resistance
significantly deteriorate. Thus, the Mn content is 10.0% or less. The Mn
content is preferably 9.0% or less. A strengthening property of Mn in the
heat-treated steel material including martensite as its main structure is
about 100 MPa/mass %, which is about 2.5 times a strengthening property
of Mn in a steel material including ferrite as its main structure (about
40 MPa/mass %).

[0092] (Cr: 0.01% to 1.00%)

[0093] Cr is an element which enhances the hardenability of the steel
sheet for heat treatment, thereby enabling to stably obtain the strength
of the heat-treated steel material. If the Cr content is less than 0.01%,
there is a case where the effect by the above function cannot be
sufficiently obtained. Thus, the Cr content is 0.01% or more. The Cr
content is preferably 0.02% or more. On the other hand, if the Cr content
exceeds 1.00%, Cr concentrates in carbides in the steel sheet for heat
treatment, resulting in that the hardenability lowers. This is because,
as Cr concentrates, the carbides are more stabilized, and the carbides
are less solid-soluble during heating for quenching. Thus, the Cr content
is 1.00% or less. The Cr content is preferably 0.80% or less.

[0094] (Ti: 0.010% to 0.100%)

[0095] Ti has an effect of greatly improving the toughness of the
heat-treated steel material. Namely, Ti suppresses recrystallization and
further forms fine carbides to suppress grain growth of austenite during
heat treatment for quenching at a temperature of an Ac.sub.3 point or
higher. Fine austenite grains are obtained by the suppression of the
grain growth, resulting in that the toughness greatly improves. Ti also
has an effect of preferentially bonding with N in the steel sheet for
heat treatment, thereby suppressing B from being consumed by the
precipitation of BN. As will be described later, B has an effect of
improving the hardenability, so that it is possible to securely obtain
the effect of improving the hardenability by B through suppressing the
consumption of B. If the Ti content is less than 0.010%, there is a case
where the effect by the above function cannot be sufficiently obtained.
Thus, the Ti content is 0.010% or more. The Ti content is preferably
0.015% or more. On the other hand, if the Ti content exceeds 0.100%, a
precipitation amount of TiC increases so that C is consumed, and
accordingly, there is a case where the heat-treated steel material cannot
obtain sufficient strength. Thus, the Ti content is 0.100% or less. The
Ti content is preferably 0.080% or less.

[0096] (B: 0.0010% to 0.0100%)

[0097] B is a very important element having an effect of significantly
enhancing the hardenability of the steel sheet for heat treatment. B also
has an effect of strengthening a grain boundary to increase the toughness
by segregating in the grain boundary. B also has an effect of improving
the toughness by suppressing the grain growth of austenite during heating
of the steel sheet for heat treatment. If the B content is less than
0.0010%, there is a case where the effect by the above function cannot be
sufficiently obtained. Thus, the B content is 0.0010% or more. The B
content is preferably 0.0012% or more. On the other hand, if the B
content exceeds 0.0100%, a large amount of coarse compounds precipitate
to deteriorate the toughness of the heat-treated steel material. Thus,
the B content is 0.0100% or less. The B content is preferably 0.0080% or
less.

[0098] (Si: 0.08% or less)

[0099] Si is not an essential element, but is contained in the steel as
impurities, for example. The higher the Si content, the higher a
temperature at which austenite transformation occurs. As this temperature
is high, a cost required for heating for quenching increases, or
quenching is likely to be insufficient due to insufficient heating.
Besides, as the Si content is high, wettability and alloying
processability of the steel sheet for heat treatment are lowered, and
therefore stability of hot-dip process and alloying process deteriorates.
Therefore, the lower the Si content, the better. In particular, when the
Si content exceeds 0.08%, the temperature at which austenite
transformation occurs is noticeably high. Thus, the Si content is 0.08%
or less. The Si content is preferably 0.05% or less.

[0100] (P: 0.050% or less)

[0101] P is not an essential element, but is contained in the steel as
impurities, for example. P deteriorates the toughness of the heat-treated
steel material. Therefore, the lower the P content, the better. In
particular, when the P content exceeds 0.050%, the toughness noticeably
lowers. Thus, the P content is 0.050% or less. The P content is
preferably 0.005% or less. It requires a considerable cost to decrease
the P content to less than 0.001%, and it sometimes requires a more
enormous cost to decrease the P content to less than 0.001%. Thus, there
is no need to decrease the P content to less than 0.001%.

[0102] (S: 0.0500% or less)

[0103] S is not an essential element, but is contained in the steel as
impurities, for example. S deteriorates the toughness of the heat-treated
steel material. Therefore, the lower the S content, the better. In
particular, when the S content exceeds 0.0500%, the toughness noticeably
lowers. Thus, the S content is 0.0500% or less. The S content is
preferably 0.0300% or less. It requires a considerable cost to decrease
the S content to less than 0.0002%, and it sometimes requires a more
enormous cost to decrease the S content to less than 0.0002%. Thus, there
is no need to decrease the S content to less than 0.0002%.

[0104] (N: 0.0100% or less)

[0105] N is not an essential element, but is contained in the steel as
impurities, for example. N contributes to the formation of a coarse
nitride and deteriorates local deformability and the toughness of the
heat-treated steel material. Therefore, the lower the N content, the
better. In particular, when the N content exceeds 0.0100%, the local
deformability and the toughness noticeably lower. Thus, the N content is
0.0100% or less. It requires a considerable cost to decrease the N
content to less than 0.0008%. Thus, there is no need to decrease the N
content to less than 0.0008%. It sometimes requires a more enormous cost
to decrease the N content to less than 0.0002%.

[0106] Ni, Cu, Mo, V, Al, and Nb are not essential elements, but are
optional elements which may be appropriately contained, up to a specific
amount as a limit, in the steel sheet for heat treatment and the
heat-treated steel material.

[0108] Ni, Cu, Mo, V, Al, and Nb are elements which enhance the
hardenability of the steel sheet for heat treatment, thereby enabling to
stably obtain the strength of the heat-treated steel material. Thus, one
or any combination selected from the group consisting of these elements
may be contained. However, if the Ni content exceeds 2.0%, the effect by
the above function saturates, which only increases a wasteful cost. Thus,
the Ni content is 2.0% or less. If the Cu content exceeds 1.0%, the
effect by the above function saturates, which only increases a wasteful
cost. Thus, the Cu content is 1.0% or less. If the Mo content exceeds
1.0%, the effect by the above function saturates, which only increases a
wasteful cost. Thus, the Mo content is 1.0% or less. If the V content
exceeds 1.0%, the effect by the above function saturates, which only
increases a wasteful cost. Thus, the V content is 1.0% or less. If the Al
content exceeds 1.00%, the effect by the above function saturates, which
only increases a wasteful cost. Thus, the Al content is 1.00% or less. If
the Nb content exceeds 1.00%, the effect by the above function saturates,
which only increases a wasteful cost. Thus, the Nb content is 1.00% or
less. In order to securely obtain the effect by the above function, each
of the Ni content, the Cu content, the Mo content, and the V content is
preferably 0.1% or more, and each of the Al content and the Nb content is
preferably 0.01% or more. Namely, it is preferable to satisfy one or any
combination of the following: "Ni: 0.1% to 2.0%", "Cu: 0.1% to 1.0%",
"Mo: 0.1% to 1.0%", "V: 0.1% to 1.0%", "Al: 0.01% to 1.00%", or "Nb:
0.01% to 1.00%".

[0109] As described above, C and Mn increase the strength of the
heat-treated steel material mainly by increasing the strength of
martensite. However, it is not possible to obtain tensile strength of
1.800 GPa or more, if the "Expression 1" is not satisfied where [C]
denotes a C content (mass %) and [Mn] denotes a Mn content (mass %).
Accordingly, the "Expression 1" should be satisfied.

4612.times.[C]+102.times.[Mn]+605.gtoreq.1800 "Expression 1";

[0110] Next, a microstructure of the heat-treated steel material according
to the present embodiment will be described. The heat-treated steel
material according to the present embodiment includes a microstructure
represented by martensite: 90 volume % or more. The balance of the
microstructure is, for example, retained austenite. When the
microstructure is formed of martensite and retained austenite, a volume
fraction (volume %) of the martensite may be measured through an X-ray
diffraction method with high accuracy. Specifically, diffracted X-rays
obtained by the martensite and the retained austenite are detected, and
the volume fraction may be measured based on an area ratio of the
diffraction curve. When the microstructure includes another phase such as
ferrite, an area ratio (area %) of the other phase is measured through
microscopic observation, for example. The structure of the heat-treated
steel material is isotropic, so that a value of an area ratio of a phase
obtained at a certain cross section may be regarded to be equivalent to a
volume fraction in the heat-treated steel material. Thus, the value of
the area ratio measured through the microscopic observation may be
regarded as the volume fraction (volume %).

[0111] Next, a dislocation density in martensite in the heat-treated steel
material according to the present embodiment will be described. The
dislocation density in the martensite contributes to the improvement of
tensile strength. When the dislocation density in the martensite is less
than 9.0.times.10.sup.15 m.sup.-2, it is not possible to obtain the
tensile strength of 1.800 GPa or more. Thus, the dislocation density in
the martensite is 9.0.times.10.sup.15 m.sup.-2 or more.

[0112] The dislocation density may be calculated through an evaluation
method based on the Williamson-Hall method, for example. The
Williamson-Hall method is described in "G. K. Williamson and W. H. Hall:
Acta Metallurgica, 1(1953), 22", "G. K. Williamson and R. E. Smallman:
Philosophical Magazine, 8(1956), 34", and others, for example.
Concretely, peak fitting of respective diffraction spectra of a {200}
plane, a {211} plane, and a {220} plane of body-centered cubic structure
is carried out, and .beta..times.cos.theta./.lamda. is plotted on a
horizontal axis, and sin.theta./.lamda. is plotted on a vertical axis
based on each peak position (.theta.) and half-width (.beta.). An
inclination obtained from the plotting corresponds to local strain
.epsilon., and the dislocation density .rho. (m.sup.-2) is determined
based on a following "Expression 2" proposed by Wlliamson, Smallman, et
al. Here, b denotes a magnitude of Burgers vector (nm).

.rho.=14.4.times..epsilon..sup.2/b.sup.2 "Expression 2"

[0113] Further, the heat-treated steel material according to the present
embodiment has the tensile strength of 1.800 GPa or more. The tensile
strength may be measured based on rules of ASTM standard E8, for example.
In this case, when producing test pieces, soaked portions are polished
until their thicknesses become 1.2 mm, to be worked into half-size
plate-shaped test pieces of ASTM standard E8, so that a tensile direction
is parallel to the rolling direction. A length of a parallel portion of
each of the half-size plate-shaped test pieces is 32 mm, and a width of
the parallel portion is 6.25 mm. Then, a strain gage is attached to each
of the test pieces, and a tensile test is conducted at a strain rate of 3
mm/min at room temperature.

[0114] Next, a method of manufacturing the heat-treated steel material,
namely, a method of treating the steel sheet for heat treatment, will be
described. In the treatment of the steel sheet for heat treatment, the
steel sheet for heat treatment is heated to a temperature zone of not
less than an Ac.sub.3 point nor more than "the Ac.sub.3 point
+200.degree. C." at an average heating rate of 10.degree. C./s or more,
the steel sheet is then cooled from the temperature zone to an Ms point
at a rate equal to or more than an upper critical cooling rate, and
thereafter, the steel sheet is cooled from the Ms point to 100.degree. C.
at an average cooling rate of 50.degree. C./s or more.

[0115] If the steel sheet for heat treatment is heated to the temperature
zone of the Ac.sub.3 point or more, the structure becomes an austenite
single phase. If the average heating rate is less than 10.degree. C./s,
there is a case that an austenite grain becomes excessively coarse, or
the dislocation density lowers due to recovery, thereby deteriorating the
strength and the toughness of the heat-treated steel material. Thus, the
average heating rate is 10.degree. C./s or more. The average heating rate
is preferably 20.degree. C./s or more, and more preferably 50.degree.
C./s or more. When the reaching temperature of the heating exceeds "the
Ac.sub.3 point +200.degree. C.", there is a case that an austenite grain
becomes excessively coarse, or the dislocation density lowers, thereby
deteriorating the strength and the toughness of the heat-treated steel
material. Thus, the reaching temperature is "the Ac.sub.3 point
+200.degree. C." or less.

[0116] The above-described series of heating and cooling may also be
carried out by, for example, a hot stamping method, in which heat
treatment and hot forming are conducted concurrently, or high-frequency
heating and quenching. The period of time of retention of the steel sheet
in the temperature zone of not less than the Ac.sub.3 point nor more than
"the Ac.sub.3 point +200.degree. C." is preferably 30 seconds or more,
from a viewpoint of increasing the hardenability of steel by accelerating
the austenite transformation to dissolve carbides. The retention time is
preferably 600 seconds or less, from a viewpoint of productivity.

[0117] If the steel sheet is cooled from the temperature zone to the Ms
point at the rate equal to or more than the upper critical cooling rate
after being subjected to the above-described heating, the structure of
the austenite single phase is maintained, without occurrence of diffusion
transformation. If the cooling rate is less than the upper critical
cooling rate, the diffusion transformation occurs so that ferrite is
easily generated, resulting in that the microstructure in which the
volume fraction of martensite is 90 volume % or more is not be obtained.
Thus, the cooling rate to the Ms point is equal to or more than the upper
critical cooling rate.

[0118] If the steel sheet is cooled from the Ms point to 100.degree. C. at
the average cooling rate of 50.degree. C./s or more after the cooling to
the Ms point, the transformation from austenite to martensite occurs,
resulting in that the microstructure in which the volume fraction of
martensite is 90 volume % or more can be obtained. As described above,
the transformation from austenite to martensite is accompanied by
expansion, so that in accordance with the martensite transformation,
strain (transformation strain) is introduced into surrounding
non-transformed austenite, and in order to lessen the transformation
strain, the martensite right after the transformation undergoes
supplemental deformation. Concretely, the martensite undergoes slip
deformation while being subjected to introduction of dislocations.
Consequently, the martensite includes high-density dislocations. In the
present embodiment, the specific amounts of C and Mn are contained, so
that the dislocations are generated in the martensite at extremely high
density, and the dislocation density becomes 9.0.times.10.sup.15 m.sup.-2
or more. If the average cooling rate from the Ms point to 100.degree. C.
is less than 50.degree. C./s, recovery of dislocations easily occurs in
accordance with auto-tempering, resulting in that the dislocation density
becomes insufficient and the sufficient tensile strength cannot be
obtained. Thus, the average cooling rate is 50.degree. C./s or more. The
average cooling rate is preferably 100.degree. C./s or more, and more
preferably 500.degree. C./s or more.

[0119] In the manner as described above, the heat-treated steel material
according to the present embodiment provided with the excellent toughness
and weldability, and the tensile strength of 1.800 GPa or more, can be
manufactured. An average grain diameter of prior austenite grains in the
heat-treated steel material is about 10 .mu.m to 20 .mu.m.

[0120] A cooling rate from less than 100.degree. C. to the room
temperature is preferably a rate of air cooling or more. If the cooling
rate is less than the air cooling rate, there is a case that the tensile
strength lowers due to the influence of auto-tempering.

[0121] It is also possible to perform hot forming such as the hot stamping
described above, during the above-described series of heating and
cooling. Specifically, the steel sheet for heat treatment may be
subjected to forming in a die before the temperature of the steel sheet
reaches the Ms point after the heating to the temperature zone of not
less than the Ac.sub.3 point nor more than "the Ac.sub.3 point
+200.degree. C.". Bending, drawing, bulging, hole expansion, and flanging
may be cited as examples of the hot forming. These belong to press
forming, but, as long as it is possible to cool the steel sheet in
parallel with the hot forming or right after the hot forming, hot forming
other than the press forming, such as roll forming, may also be
performed.

[0122] The steel sheet for heat treatment may be a hot-rolled steel sheet
or a cold-rolled steel sheet. An annealed hot-rolled steel sheet or an
annealed cold-rolled steel sheet obtained by performing annealing on a
hot-rolled steel sheet or a cold-rolled steel sheet may also be used as
the steel sheet for heat treatment.

[0123] The steel sheet for heat treatment may be a surface-treated steel
sheet such as a plated steel sheet. Namely, a plating layer may be
provided on the steel sheet for heat treatment. The plating layer
contributes to improvement of corrosion resistance and the like, for
example. The plating layer may be an electroplating layer or a hot-dip
plating layer. An electrogalvanizing layer and a Zn--Ni alloy
electroplating layer may be cited as examples of the electroplating
layer. A hot-dip galvanizing layer, an alloyed hot-dip galvanizing layer,
a hot-dip aluminum plating layer, a hot-dip Zn--Al alloy plating layer, a
hot-dip Zn--Al--Mg alloy plating layer, and a hot-dip Zn--Al--Mg--Si
alloy plating layer may be cited as examples of the hot-dip plating
layer. A coating amount of the plating layer is not particularly limited,
and may be a coating amount within an ordinary range, for example.
Similarly to the steel sheet for heat treatment, the heat-treated steel
material may be provided with a plating layer.

[0124] Note that any one of the above-described embodiments only presents
concrete examples in carrying out the present invention, and the
technical scope of the present invention should not be construed in a
limited manner by these. That is, the present invention may be embodied
in various forms without departing from its technical idea or its main
feature.

EXAMPLES

[0125] Next, experiments conducted by the inventors of the present
application will be described.

[0126] In the experiment, slabs each including a chemical composition
presented in Table 1 were subjected to hot-rolling and cold-rolling, to
thereby manufacture cold-rolled steel sheets each including a thickness
of 1.4 mm, as steel sheets for heat treatment. Blank columns in Table 1
indicate that contents of elements in the blank columns are less than
detection limits, and the balance is Fe and impurities. Underlines in
Table 1 indicate that the underlined numerical values are out of the
ranges of the present invention.

[0127] Then, samples each including a thickness of 1.4 mm, a width of 30
mm, and a length of 200 mm were produced from the respective cold-rolled
steel sheets, and the samples were heated and cooled under conditions
presented in Table 2. The heating and cooling imitate heat treatment in
hot forming. The heating in the experiment was performed by energization
heating. After the cooling, soaked portions were cut out from the
samples, and the soaked portions were subjected to a tensile test and an
X-ray diffraction test.

[0128] The tensile test was conducted based on rules of ASTM standard E8.
In the tensile test, a tensile tester made by Instron corporation was
used. When preparing test pieces, soaking portions were polished until
their thicknesses became 1.2 mm, to be worked into half-size plate-shaped
test pieces of ASTM standard E8, so that a tensile direction was parallel
to the rolling direction. A length of a parallel portion of each of the
half-size plate-shaped test pieces was 32 mm, and a width of the parallel
portion was 6.25 mm. Then, a strain gage was attached to each of the test
pieces, and a tensile test was conducted at a strain rate of 3 mm/min at
room temperature. As the strain gage, KFG-5 (gage length: 5 mm) made by
KYOWA ELECTRONIC INSTRUMENTS CO., LTD. was used.

[0129] In the X-ray diffraction test, portions up to a depth of 0.1 mm
from surfaces of the soaked portions were chemically polished by using
hydrofluoric acid and a hydrogen peroxide solution, thereby preparing
test pieces for the X-ray diffraction test each having a thickness of 1.1
mm. Then, a Co tube was used to obtain an X-ray diffraction spectrum of
each of the test pieces in a range of 20 from 45.degree. to 130.degree.,
and a dislocation density was determined from the X-ray diffraction
spectrum. Further, volume fractions of martensite were also determined
based on the detection results of the diffracted X-rays and results of
observation by optical microscope according to need in addition to the
results of the diffracted X-rays.

[0130] The dislocation density was calculated through the evaluation
method based on the above-described Williamson-Hall method. Concretely,
in this experiment, peak fitting of respective diffraction spectra of a
{200} plane, a {211} plane, and a {220} plane of body-centered cubic
structure was carried out, and .beta..times.cos.theta./.lamda. was
plotted on a horizontal axis and sin.theta./.lamda. was plotted on a
vertical axis based on each peak position (.theta.) and half-width
(.beta.). Then, the dislocation density .rho. (m.sup.-2) was determined
based on the "Expression 2".

[0131] Results of these are presented in Table 2. Underlines in Table 2
indicate that the underlined numerical values are out of the ranges of
the present invention.

[0132] As presented in Table 2, in the samples No. 1 to No. 6, No. 10 to
No. 13, and No. 16 to No. 20, since the chemical compositions were within
the ranges of the present invention, and the manufacturing conditions
were also within the ranges of the present invention, desired
microstructures and dislocation densities were obtained in the
heat-treated steel materials. Further, since the chemical compositions,
the microstructures, and the dislocation densities were within the ranges
of the present invention, the tensile strengths of 1.800 GPa or more were
obtained.

[0133] In the samples No. 7 to No. 9, No. 14, No 15, No. 21, and No. 22,
although the chemical compositions were within the ranges of the present
invention, the manufacturing conditions were out of the ranges of the
present invention, and thus it was not possible to obtain desired
dislocation densities. Further, since the dislocation densities were out
of the ranges of the present invention, the tensile strengths were low to
be less than 1.800 GPa.

[0134] In the samples No. 23 and No. 24, since the Mn contents were out of
the ranges of the present invention, even though the manufacturing
conditions were within the ranges of the present invention, the
dislocation densities were less than 9.0.times.10.sup.15 m.sup.-2, and
the tensile strengths were low to be less than 1.800 GPa.

[0135] In the sample No. 25, since the C content was out of the range of
the present invention, even though the manufacturing condition was within
the range of the present invention, the dislocation density was less than
9.0.times.10.sup.15 m.sup.-2, and the tensile strength was low to be less
than 1.800 GPa.

[0136] In the sample No. 26, the "Expression 1" was not satisfied, so that
even when the manufacturing condition was within the range of the present
invention, the dislocation density was less than 9.0.times.10.sup.15
m.sup.-2, and the tensile strength was low to be less than 1.800 GPa.

[0137] From these results, it is understood that it is possible to obtain
a high-strength heat-treated steel material according to the present
invention. Further, according to the present invention, it is not
required that C is contained to such an extent as to deteriorate the
toughness and the weldability in order to obtain the high strength, so
that it is also possible to obtain excellent toughness and weldability.

INDUSTRIAL APPLICABILITY

[0138] The present invention may be used in the industries of
manufacturing heat-treated materials and the like used for automobiles,
for example, and in the industries of using them. The present invention
may also be used in the industries of manufacturing other mechanical
structural components, the industries of using them, and the like.